Pressure-relief valve with stepped or double piston

ABSTRACT

A pressure-limiting valve having high flow rates, especially for uses as a safety valve with mine props and the like, uses a tubular main-valve member whose interior communiciates with the valve inlet and has lateral bores opening into the valve outlet when the bores are shifted past a sealing ring in the valve housing. A pilot-valve body is received in the tubular valve and the valve spring bears upon the tubular valve. The main valve is shifted by differential pressure.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase of PCT/DE90/00332 filed May 10,1990 based upon German national applications P39 22 894.0 filed July 12,1989, P39 29 094.8 filed Sept. 1, 1989 and P39 16 260.5 filed May 19,1989 under the International Convention.

1. Field of the Invention

The invention relates to a pressure-relief valve for the protection ofhydraulic units, particularly the hydraulic face support systems inunderground mining and tunnel construction, against sudden overload dueto rock pressure or similar causes, with a valve piston arranged toslide longitudinally in the valve housing against the force of a springand with an inlet opening on the one side of the piston provided withblind bore and with radial bores at the extremity, as well as withoutlet holes on the other side of the valve piston, whereby the valvepiston is sealed by means of an O-shaped annular seal over which theradial bores can travel.

2. Background of the Invention

Such pressure-relief valves, also known as safety valves, are used wherea hydraulic system, particularly a system of hydraulic struts (props) inunderground coal mining, can be damaged by excessive pressure. Thesehydraulic struts are arranged either as individual struts or integratedin a shield-type support. Because of the constant danger, there areofficial requirements for providing pressure-relief valves not only forsuch shield-type support systems, but also for the individual hydraulicstruts, in order to prevent permanent damage or even destructionendangering the miners, in cases of overload. DE-OS 28 30 891 describesa pressure-relief valve wherein excessive pressure in the hydraulicsystem is reduced by means of a valve spring braced between the lockingscrew and the valve piston. On the valve spring cap or the valve springretainer a conically or spherically shaped valve-closing body isprovided which is lifted off the valve seat in case of overload. On thepiston a damping cylinder is provided, which limits the flow passageopening. However, such valves do not afford the locking safety requiredfor the use as pressure-relief valves or safety valves in mining.Furthermore an accurate positioning particularly of the spring is verydifficult, which means that all of the requirements for a safe responseof such a mining safety valve are not fulfilled.

DE-OS 33 14 837 described another pressure-relief valve wherein thevalve spring is arranged in the valve housing so that it presses on thevalve spring cap and thereby on the valve piston and this way influencesthe opening of the valve corresponding to the setting. The valve pistonis slidably guided on a piston-pin bore formed in the guide, whereby thenecessary sealing is provided by an O-shaped annular seal located in agroove. In order to insure reliable operation and a timely response ofthe valve while ensuring a long life for the sealing rings, the latterhave to be completely overridden by the radial bores of the piston.Through the radial bores, the blind hole provided in the valve pistonand connected with the piston-pin bore can be reached, so that when theO-shaped sealing rings are overridden the pressure medium can exit thehydraulic unit to be protected. The drawback of these knownpressure-relief valves are the low throughflow values of 40 to 60(maximum 100 ) liters per minute. This is not satisfactory for therequired safety and quick response of such valves. Besides, it isdisadvantageous that the valve springs involved have to use wire ofconsiderable strength and have considerable coil diameter in order tocounteract the pressures; the spring size requires correspondingly largeoverall valve dimensions. The larger the throughflow, the bigger thevalve springs, and thereby the entire valve housing, have to be. In thiscase, particularly in underground mining, it is not possible to use verylarge valves, not to mention that the required cross sections for theevacuation of the pressure medium would not be available.

OBJECT OF THE INVENTION

It is therefore the object of the present invention to provide apressure-relief valve with large throughflow (over 1000 liter perminute), but having small overall dimensions (valve spring, springspace) and workable closing values.

SUMMARY OF THE INVENTION

According to the invention, this object is achieved in that the valvepiston is designed as a continuous tube shell with a large dischargesurface having an attachment with a reduced diameter serving as acontrol piston and also sealing rings on both sides of the outletopening designed as a cross bore, thereby defining two separate chamberssubject to the equal pressure inside and above the valve piston and thatthe control piston rests against the spring cap loaded by the valvespring.

Based on this integral construction, it is possible to design thecontrol piston with such a small diameter, that first of all favorablydifferentiated surfaces result, and in addition the valve springassigned to this control piston can also have very favorable values,i.e. small dimensions. Based on a valve spring with smaller wirethickness and corresponding spring force a compact valve housing can beused, into which the valve spring can be integrated without difficulty.Moreover, the control piston becomes the pressure-relief valve at thesame time, which in itself already allows for a throughflow capacity ofroughly 400 liter per minute. On top of that, the actual valve pistondue to its favorable dimensions, i.e. its large discharge surface, canallow for a considerably throughflow capacity of 2000 liter per minuteand more, so that altogether a valve is created which will safely afforda throughflow of 1000 liter per minute. Besides, due to the adroitdischarge of the pressure medium from the area of the control piston, animprovement of the closing value is obtained. A further improvement ofthe closing values is obtained by using such sealing rings which canhave reduced friction coefficient. By creating the chambers on bothsides of the valve piston and their connection--as a result both beingsubjected to the same pressure--a pressure balance is created, so thatthe control piston exclusively is responsible for the positioning of thespring.

According to a feature embodiment of the invention, the spring space isseparated by a partitioning wall from the inlet opening, whichunderneath the partition changes over to the cross bore serving as anoutlet opening. The valve piston forms a structural unit with thecontrol piston equipped with an inner blind bore starting out from theblind bore and with radial bores and which is slidably guided in a borewith an O-shaped sealing ring made in the partitioning wall. The chamberare connected over passage bores made in the lid of the valve piston. Ina construction of this type, it is possible to rely on stable movableparts and on a design of the control piston such as found in provenvalves. The stepped piston according to the invention provides thedifferential areas which are required for the sought low closing values.

A favorable design is achieved by centrally introducing the piston inthe lid and by arranging the throughflow passages around it in a circle.This way the compact construction of the valve piston and control pistonis preserved, since the control piston can be made for instance byturning (machining) off the valve piston in its upper region. Thethroughflow passages make sure that the valve piston maintains itsbalance under normal pressure circumstances, so that the cross bore iseffectively closed. Only when the pressure increases can the smallcontrol piston fulfill its function, by pushing the valve springretainer against the force of the valve spring, thereby alsosimultaneously moving the valve piston, so that pressure medium can exitthe valve at the same time through the area of the valve piston and ofthe control piston. Thereby the necessary relief of the hydraulic unitis achieved, and namely as long as it takes to eliminate the excessivepressure, whereafter the pressure-relief valve closes immediately. Thequick and accurate response is insured due to the fact that the controlpiston has an outer annular groove in the area of its onset.

In order to reduce and moderate the discharge resistance it is providedthat the cross bores start out from an annular channel and run outwardlyinclined. This way, a rerouting of the flow of pressure medium isreduced to a minimum.

The already mentioned large throughflow quantities are reached byadjusting the diameters of the valve piston and control piston, wherebya favorable embodiment is one in which valve piston has an externalradius of 25 mm while the external radius of the control piston is 10mm. Further it is proposed to provide the valve piston with 12 radialbores with a diameter of 5 mm and the control piston with 6 radial boreshaving 2.5 mm., so that the corresponding amounts of pressure medium canbe safely evacuated.

Surprisingly favorable closing values are achieved with thepressure-relief valve of the invention when the sealing rings,optionallyalso the O-shaped annular seal, are made of Teflon plastic material TFM1600 and have a rectangular cross section. Such sealing rings, which arenot very flexible, still insure an effective sealing, have extremelylong operational lives and permit the achievement of the alreadymentioned favorable closing values , because the friction betweensealing rings and valve piston is reduced to a minimum. This isparticularly the case when the sealing rings have a rectangular crosssection. Such sealing rings lead to a good surface sealing, withoutsignificant reduction of the movements of the valve piston due to thegenerated friction. In this way it is possible to use continuous tubularshells with very large diameters for valve pistons, which contributes toan increase of the admissible and possible throughflowing amounts. Inorder to mount the teflon sealing rings quickly and accurately it isproposed that the sealing rings be arranged in upwardly or downwardlyopening sealing groves and be mounted via a plug-in socket or a guidescrew, which at the same time can constitute the partition. The plug-insocket is advantageously exposed to the pressure of the pressure mediumand secured in the connection piece, without requiring a locking orbolting. By contrast, the guide screw is screwed in from the springspace and thereby effectively fastens the upper sealing ring, which willnot be removed from its position in the assigned annular groove duringoperation.

The control piston can cut across the partitioning wall of the valvepiston shaped like a tubular shell and can be indented therewith in theopening direction of the valve as a stepped indented piston withdifferential surfaces. Here, the differential surfaces in the transitionarea between the control piston and the valve piston can be preselected,whereby due to the indentation of the valve piston and the controlpiston it is insured that both perform the same respective movements, orthat the control piston respectively entrains the valve piston andforces it into the respective position. The differential surfaces areparticularly predetermined by the fact that valve piston continues witha kind of ram, whereby the pressure medium usually enters the blind boreof the valve piston, than further into the inner blind bore of thecontrol piston and from there over the radial bores into the springspace, so that here too on both sides of the cross bores the samepressure is built up. The differential surface, i.e. the pressuresurface is thereby determined only by the already mentioned ram, orrather guide attachment. Irregular movements of the control piston andvalve piston are precluded by the fact that the control piston issupported in the partitioning wall of the valve cylinder and guided inthe adjusting screw.

In another embodiment, the valve piston is formed as a tubular shellwith closed partition wall the chambers can be built like annularchannels and mutually connected through housing bores running parallellyto the valve axis. The diameter of the tubular can be reduced in thedirection of the opening. Here it is not necessary to displace largeamounts of pressure medium, but only the amounts present in the annularchannels, whereby it is still possible for the valve to respond quicklyand accurately with favorable spring dimensions. The reduced wirediameters of the valve spring permit favorable overall dimensions,whereby it is insured by this particular construction that theadditional channels can be laid out so that the dimensions of the valvedo not have to suffer. The annular channels and the connecting housingbores are so set up that neither the diameter or width of thepressure-relief valve nor the length of the valve housing have to bechanged.

A connection to the prop in the case of individual prop valves is easilyand quickly done, since the connection piece has an external threadedconnection with the inlet opening.

Also in such embodiments the sealing with respect to the connectedhydraulic component can be done simply, by providing the connectionpiece on its free surface with a circular groove and an externalO-shaped annular seal.

Instead of the housing bypass bores and the annular channels it is alsopossible to have a throttle bore in the partition wall and the diameterreduction in the transition area between the partition and the springspace can be effected on the tubular shell. Based on these specialfeatures it is possible to make available large bores for the passage oflarge amounts of pressure medium, just like in the case of the otherembodiments described. Since the spring space and the antechamber areconnected by a throttle bore, on both sides of the partition wall thesame pressure is established. Starting from this point, when an overloadoccurs a quick overriding of the 0-ring respectively sealing ring by theradial bores and therewith a quick exit of the pressure medium isinsured, whereby springs with a flat characteristic curve can be used,so that one can operate with relatively small and compact springs. Thediameter reduction insures reliable displacement of the valve pistonagainst the force of the valve spring showing a clearly favorablecharacteristic spring curve. At the same time, based on thisconstruction the retraction of the valve piston is insured when theexcessive load ceases to exist, thereby also insuring the response ofthe hydraulic system employed for support. The response of the valvepiston is particularly insured when the diameter reduction results in apressure-surface difference of 0.08 to 0.2 cm.

The penetration of dirt into the valve through the cross bores isprevented by providing in the external wall of the valve housing in thearea of the cross bores a wide annular groove with an embedded sealingring.

The result is as highly compact pressure-relief valve allowing largeramounts of throughflow, i.e. with a throughflow of 1000 to 3000 literper minute, which, based on its favorable overall dimensions, can beemployed practically everywhere in underground mining, as well as inother fields. This is achieved with only digit increase in manufacturingcost. The favorable overall dimensions result primarily from the factthat springs with a slowly ascending characteristic curve are used,which insure a good response of the pressure-relief valve in conditionsof very low space requirements. The functioning of the valve is insuredby advantageous closing values, by using correspondingly suitablesealing rings. The use of a valve piston with very large dischargesurface insures the amounts of the throughflow of 1000, 2000 and moreliter per minute, so that the special conditions existing in undergroundmining are taken into consideration.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a cross-sectional view of a pressure-relief valve inlongitudinal section with stepped piston, in open and closed position;

FIG. 2 is a similar view of a pressure-relief valve with a two-partpiston;

FIG. 3 is another axial section is a pressure-relief valve with a valvepiston designed as a tubular shell and lateral housing bores; and

FIG. 4 is an axial cross-sectional view of a pressure-relief valve withsimple valve piston and laterally closed cross bores.

SPECIFIC DESCRIPTION

FIG. 1 shows a pressure-relief valve 1 in its two possible positions. Onthe right side the open position and on the left side the closedposition of the pressure-relief valve 1 are shown. The valve housing 2is shown in section, so that the individual components are recognizable.

The valve housing 2 consists of an upper portion 3 with an inner thread4 and connection piece 5 having an external thread 6. The externalthread 6 is approximately aligned with the external connection thread 9,by means of which the pressure-relief valve can be connected, i.e.screwed on to for instance a hydraulic nine prop strut.

The connection piece 5 has an internal bore which at the same time isthe inlet bore 13 for the pressure medium. In this inlet bore 13 thevalve piston 10 is inserted. The piston is formed as a tubular shell 20.The valve piston 10 has a blind bore 14 which continues with the radialbores 15, 16. In the closed position of the valve, the radial bores 15,16 are separated by the sealing ring 19 from the annular channel 18 andthe cross bores 17. The pressure medium entering the pressure-reliefvalve from the inlet bore 13 can not exit via the blind bore 14 and theradial bores 15, 16 when the valve is closed. The lower edge (28) of thecross bore 17 is located, as shown in the left portion of FIG. 1 abovethe radial bores 15, 16 sealed off, or separated by sealing ring 19.

The outer wall 29 of the valve piston 10 or the tubular shell 20 issmooth all the way, so that when it rides over the sealing ring 19normal friction occurs.

On the piston valve 10 centrally and thus along the valve axis 21 acontrol piston 60 is formed, which is spring loaded against the openingdirection 61 by the valve spring 11. The valve spring 11 rests on theone hand on the adjusting screw 12 and on the other hand on the springcap 26, the control piston 60 resting against the latter. The valvespring 11 arranged in the spring space 22 can, in this way, efficientlyload the control piston 60 so that this piston together with valvepiston 10 can open only when the set spring force is surpassed. Thesetting of the valve spring takes place by means of the adjusting screw12, which can be adjusted in the head portion 38 in the inner bore 39via the thread 40. It has an inner hexagonal socket head 44 in order tosimplify the adjusting operation.

The sealing of the valve piston 10 in the form of a tubular shell 20takes place first of all, as already mentioned, via the sealing ring 19lodged in an annular groove 55 and then also via the upper sealing ring51 lodged in the annular groove 50. These sealing rings are made of aplastic material Teflon generating as little friction as possible.

The pressure medium entering the pressure-relief valve 1 through theinlet bore 13 flows via the blind bore 14 up to the upper end of thevalve piston 10 represented by the lid 79. This lid 79 is provided withpassage openings 80, 81, so that the pressure medium in the blind boreis present in chamber 47 as well as in the overlying chamber 46, and inthis way is present on both sides of the valve piston 10. Thus, thevalve piston 10 floats in the position shown in the left half of FIG. 1,as long as the pressure conditions set by means of the valve spring 11are maintained.

At the same time, the pressure medium flows through the control piston60, which has an inner blind bore 62 and terminally arranged radialbores 63, 64. It is sealed via O-ring 75, so that only when this O-ring75 is overridden can pressure medium enter the spring space 22 viaradial bores 63, 64. For this to happen, the force of the valve spring11 has to be overcome. Since the external wall 65 of the control piston60 is smooth and the O-ring 75 has a relatively small diameter, here toothe friction forces are so small that they do not impair the closingvalues of the valve.

When the pressure-relief valve 1 is triggered, the pressure medium flowsthrough the inlet opening 13, the blind bore 14 and the inner blind bore62, as well as through the radial bores 63, 64 into the spring space 22.In order to prevent a backflow and at the same time to increase thethroughflow capacity of such a pressure-relief valve, the adjustingscrew 12 has a passage opening 68, through which the pressure medium canbe discharged. In the embodiment of FIG. 1, this passage opening 68 isclosed by a dirt-protection valve. This dirt-protection valve consistsof a sealing bolt 95 having a seat surface 96, so that--as can be seenfrom FIG. 1 --when the sealing bolt 95 is under the effect of the forceof spring 100 an effective sealing is accomplished. The bolt stem 97extends into the spring space 22 and is equipped with a disk washer 98.The spring 100 is then tightened between this disk washer 98 and theinside 101 of the adjusting screw 12, so that the sealing bolt 95remains in the position shown in the left side of FIG. 1 when thepressure-relief valve 1 is closed. When the pressure-relief valve 1 istriggered and the pressure medium flows into spring space 22, thesealing bolt 95 is displaced, namely against the force of spring 100.Then the pressure medium can exit through the passage bore 68. The flowof pressure medium is thereby hindered as little as possible by the diskwasher 98 because it is provided with openings 99. The spring 100 itselfis secured by the guide 102, so that an entanglement or mutual influencebetween the valve spring 11 and the spring 100 is not possible.

In the onset area 82 of the control piston 60 an external annular groove83 is provided, which considerably improves the response of the valveduring the return stroke of the valve.

The sealing rings 19, 51 rub against the outer wall 29 of the valvepiston 10 in order to keep the valve sealed. When the valve piston 10 isdisplaced, this friction leads intentionally to an impairment, of courseonly a slight impairment, of the course of motion, since the sealingrings 19, 51 are made of Teflon or another similar plastic material.

The lower sealing ring 19 lodged in the annular groove 55 can be easilymounted in spite of its low flexibility, since in the inlet bore 13plug-in socket 85 is introduced from the opening direction 61. Thisplug-in socket 85 is secured downwards by the O-ring 86 and the supportring 87 and upwards by the O-ring 88 and the support ring 89. The uppersealing ring 51 can be mounted in a simple way, since it is secured bythe guide screw 90 which is sealed via the O-ring 91 and the supportring 92. The mounting of the guide screw 90 is facilitated due to theopening 93 which makes possible the screwing into the thread without anyproblems. Besides, the opening 78 of the inlet bore 13 is widened tosuch an extent that the plug-in socket 85 can also be introduced just assimply as the guide screw 90 can be screwed in place.

FIG. 1 shows in its right half an open pressure-relief valve 1, fromwhich it will be clear that the pressure medium, as a result forinstance of the overload in the connected hydraulic system, can now flowwithout impediment through the inlet bore 13 into the pressure-reliefvalve 1, and reach the annular channel 18 via blind bore 14, and theradial bores 15, 16. From there it is discharged the cross bores 17 fromthe pressure-relief valve. Based on the relatively large diameter of thevalve piston 10, which has an outer radius of 25 mm and on thehere-arranged 12 radial bores 15, 16 each having a diameter of 5 mm, athroughflow capacity of over 2000 liter per minute is insured. At thesame time the pressure mediums streams through the control piston 60 andits inner blind bore 62 all the way through the radial bores 63, 64 intothe spring space 22. From here it flows outside after the sealing bolt95 is lifted in the area of the adjusting screw and traversing thepassage opening 68. When the pressure surge is eliminated, the valvespring 11--through the sealing rings 19, 51 and the O-ring 75--pressesthe control piston 60 and the valve piston 10 back into their positionseen in the left half of FIG. 1. At this point, the pressure-reliefvalve is closed again and the safety of the connected hydraulic systemis insured.

In the embodiment of the pressure-relief valve as shown in FIG. 2, thevalve piston 10 and the control piston 60 form again a motion unit. Thecontrol piston 60 is indented with the valve piston 10 in the openingdirection 61 so that both respectively perform the same motions.

The control piston 60 is provided with an inner blind bore 62 succeedingthe blind bore 14 and also with terminally arranged radial bores 63, 64,so that the pressure medium can enter the spring space 22 which in thiscase coincides with the upper chamber 46. Thereby, the valve piston 10is in a state of equilibrium, since here too the same pressureconditions exist in both chamber 46 and chamber 47.

The valve spring 11 is lodged between the outer wall 65 of the controlpiston 60 and the inner wall 37 of the valve piston 10. The differentialsurface 66 is created as a result of the fact that the control piston 60is provided with a guide extension 67 which is fixed to the inner pistonforming an inner chamfer 71 and which reaches into the passage bore 68of the adjusting screw 12. Here, the guidance is achieved by means ofguide extension 67. Here too the O-ring 75 takes care of the necessarysealing. The difference in the position of the safety plate 70 which islifted together with the control piston 60 indicates whether thepressure-relief valve 1 has responded or it is out of order. The safetyplate 70 can then each time be repositioned, so that at the next use itcan again indicate whether the valve operates.

The connection between the control piston 60 and the valve piston 10 isachieved by a projection 72 in the control piston and a correspondingrecess 73 in the valve piston. When a corresponding fit is establishedit is insured that both components perform the same motion.

The O-ring 75 is secured by the retaining nut 74 screwed into acorresponding thread. On its part, the adjusting nut 12 is sealed viathe O-ring 76.

Also in the embodiment shown in FIG. 3 the valve housing 2 consists ofan upper part 3 with inner thread 4 and the connection piece 5 with anouter connection thread 9. The screwed connection as such is secured byO-ring 7, so that the pressure medium can not exit over this screwedconnection, while the outer O-ring 8 at the lower end of the connectionpiece 5 insures a safe and sealed connection with the hydraulic unit tobe protected.

FIG. 3 shows an embodiment wherein the spring space 22 receiving thevalve spring 11 is separated by a partitioning wall 23 from theso-called antechamber 24, which means also from the chamber 47. Thepartitioning wall 23 runs at a right angle with respect to the valveaxis 21. The partitioning wall 23 can serve at the same time as thespring cap 26. However, here a separate cap is provided, which restsloosely on the partition 23.

The outer wall 29 of the tubular shell 20 has two diameters. In theclosed position of the pressure-relief valve 1, the portion with thereduced diameter 30 is located approximately in the area of the loweredge 28 of the cross bore 17.

The necessary pressure balance between the spring space 22 and theantechamber 24, i.e. intermediate space 47 and 46 is established via thetwo annular channels and the housing bores 48 connecting the two. At thesame time, in addition to the sealing ring 19 and 51 an O-ring seal 31is provided. The O-rings 31 are lodged in the annular groove 32. Theannular channel, respectively space 47 is connected via the radial bores49 with the blind bore 14 and the inlet bore 13. The wall of the radialbores 49 serve thereby at the same time for connecting the groove 55wherein the sealing ring 19 is lodged.

The inner wall 33 and the outer wall 34 of the valve housing 2 arespaced apart so that a sufficiently strong wall for the valve housing isinsured. In this wall, the housing bore 48 is provided which connectsthe two chambers 46, 47 and the annular channels. When thepressure-relief valve 1 is screwed into the hydraulic unit to beprotected, the pressure medium flows via the inlet bore 13 and the blindbore 14 and the antechamber 24, as well as via the annular channel andthe chamber 47 and the housing bore 48 into the annular channel,respectively chamber 46. In this way, the same pressure is establishedon both sides, which makes possible a correspondingly favorableselection of the wire strength for the required valve spring 11.

A displacement of the valve piston 10 when excessive pressure occurs ismade possible due to the diameter reduction 30 as to the area of theouter wall 29. As already explained in the closed position of thepressure-relief valve 1, this diameter reduction 30 is located at thelower edge 28 of the bore 17, so that here a complete sealing by thesealing ring 19 is insured. With respect to the upper portion 25 thediameter reduction 30 amounts here to 25/24.7 mm.

In the adjusting screw 12 a venting bore 56 is provided, which runsaxially parallel, so that the air existing in the spring space 22 canexit without any problems when the pressure-relief valve 1 is triggered.At the same time, no underpressure is generated in this area.

In the area of the connection piece 5 the valve housing 2 has on theoutside a segment 52 with the outer connection thread 9. The externalO-ring 8 is lodged in a circular groove 54 in the free surface 53 of theconnection piece 5.

Instead of the annular channels and the housing bore, according to FIG.4 the connection between the two chambers 46, 47 and between the springface 22 and the antechamber 24 is achieved by means of a throttle bore25 which is provided in the partitioning wall 23 and which insures aconnection between the spring space 22 and the antechamber 24 via thespring cap 26 with the bore 27. Through this throttle bore 25 here alsothe same pressure is established in the spring space 22 and theantechamber 24.

A displacement of the valve piston 10 under excessive load, i.e. underincreasing pressure in the antechamber 24 is achieved due to thediameter reduction 30 in the area of the outer wall 29. In theillustrated example the rate between the diameter reduction in the lowerand the upper portion equals 25 to 24.7 mm. Based on the difference inpressure surfaces of for instance 0.11 cm valve spring 11 with clearlyadvantageous characteristic curve, namely with a shallow slope, can beconsidered. This valve spring 11 with favorable characteristic curve hassuch reduced space requirements due to the selected wire strength andits overall dimensions, that it results in advantageous dimensions forthe entire pressure-relief valve.

The sealing of the entire pressure-relief valve 1 with respect to theoutside is achieved by means of the sealing ring 36 fitted into theannular groove 35, this sealing ring covering completely the cross bores17, whereby the annular groove 35 is so deeply recessed into the outerwall 34 that the sealing ring 36 projects only slightly over the outerwall 34. This sealing system can also be applied to the embodimentsshown in FIGS. 1 to 3.

In the upper portion 38 an inner bore 39 of smaller diameter isprovided, which has a thread 40 . The adjusting screw 12 can bedisplaced without any problem so that the spring force of the valvespring 11 can be set in accordance with the circumstances. The O-ring 41provides an effective seal, whereby the O-ring can also be displacedover the sealing path 43 until it stops in front of the segment 42. Inthe embodiments shown in FIGS. 3 and 4 of the pressure-relief valve 1the closing values are less advantageous then in the embodimentsaccording to FIG. 2, and especially according to FIG. 1. Also consideredfrom the point of view of other dimensions and applicationpossibilities, the embodiment shown in FIG. 1 represents the optimalembodiment.

I claim:
 1. Pressure-relief valve for the protection of hydraulic units,particularly of hydraulic face support systems in underground mining andtunnel construction against sudden overload due to rock pressure orsimilar causes, with a valve piston defined by an outer surface arrangedto slide in a valve housing against the force of a valve spring and withan inlet opening at a first end of the valve piston provided with ablind bore therethrough and with radial bores that pierce the pistonouter surface and with cross bores provided on an inner wall of thevalve housing communicable with the radial bores, whereby the valvepiston is sealed by an annular ring overridden by the radial bores,wherein the valve piston is formed as a continuous tubular shell with anattachment at a second and remote from the first end serving as controlpiston having a reduced outer diameter relative to an outer diameter ofthe tubular shell and with sealing rings lodged in the inner wall of thevalve housing both above and below openings defining the cross bores,whereby two separate chambers are defined inside and above the valvepiston both subjected to the same pressure and the control piston restsagainst a spring cap loaded by the force of the valve spring.
 2. Thepressure-relief valve according to claim 1, wherein the control pistonis arranged centrally and upwardly intersects a cover of said valvepiston and passage openings are arranged in a core around a circle withthe passage openings capable of communicating with said chambers.
 3. Thepressure-relief valve according to claim 1 wherein the control pistonhas at least one radial bore and has an annular groove along an outerwall forming the control piston in an area below the at least one radialbores of the control piston.
 4. The pressure-relief valve according toclaim 1 wherein characterized in that the cross bores 17 run outwardlyinclined starting from an annular channel
 18. 5. Pressure-relief valveaccording to claim 1 wherein the valve piston has an outer diameter of25 mm and the control piston has an outer diameter of 10 mm.
 6. Thepressure-relief valve according to claim 1 wherein the valve piston isequipped with 12 radial bores each having a diameter of 5 mm.
 7. Thepressure-relief valve according to claim 1 wherein the sealing rings aremade of Teflon TEM 1600 and have a rectangular cross section.
 8. Thepressure-relief valve according to claim 1 wherein the sealing rings arearranged in annular grooves and are secured via a mechanism selectedfrom the group consisting of a plug-in socket and a guide screw whichmechanism at the same time serves at a partition.